1. Field of the Invention
The present invention relates to low pressure gas or mercury vapor discharge lamps and, more particularly, is concerned with apparatus and methods for cooling such low pressure gas or mercury vapor lamps.
2. Description of Related Art
Photochemical vapor deposition (photo-CVD) uses radiation to photochemically induce the deposition of thin layers on various substrates. The technique is particularly popular due to the relatively low temperatures at which deposition can be accomplished. Photo-CVD can be used to deposit thin films of selected materials onto various different substrates such as plastics, metals, glass, and composite material. This process is especially well-suited for treating numerous substrates, such as plastics, which cannot tolerate the high temperatures generally required with more conventional thermal vapor deposition techniques.
Ultraviolet (UV) radiation in the 180 nanometers (nm) to 260 nm wavelength region is commonly used in many photo-CVD processes to induce the photochemical reactions. This UV radiation is typically provided by low pressure mercury vapor lamps because they are often the cheapest and most convenient light source available which is capable of providing radiation in the required wavelength range.
Mercury vapor has emission lines at 185 nm and 254 nm. These lines carry a large percentage of the light energy emitted by an electric arc in the mercury vapor, so long as the temperature is kept below about 60.degree. C. to 70.degree. C. At higher temperatures, there is a shift in vapor emission to longer, less energetic wavelengths. These lower energy emissions are not suitable for many photo-CVD reactions. Accordingly, it is important that the temperature of the mercury vapor lamp be kept below 70.degree. C.
The cooling of low pressure mercury lamps has presented a number of problems because of substantial amount of heat is generated even during low power density operations. This problem is magnified greatly due to the added heat generated when the power density is increased to levels required for many photo-CVD processes.
A conventional low pressure mercury vapor lamp is shown at 10 in FIG. 1. The lamp 10 includes a circular tube 12 which is usually made from quartz. The tube 12 is filled with enough mercury vapor to create a maximum pressure of between about 20 to 500 millibars. Electrodes 14 and 16 provide the electric current or arc through the vapor to produce the desired UV discharge. A divider 18 is generally placed within the tube to increase the arc length without increasing the overall tube length.
Several different cooling systems have been used to cool lamps such as the one shown in FIG. 1. For example, forced air cooling is often used and provides sufficient cooling for low power density operations. Unfortunately, forced air cooling is generally not sufficient to cool mercury vapor lamps operated at high power densities. Water cooling or some other form of liquid cooling is usually required to keep high power lamps sufficiently cool. Water jackets which completely surround the lamp tube provide adequate cooling. However, water absorbs the high energy wavelengths which are necessary for photo-CVD progresses.
Attempts have been made to provide a liquid cooling jacket surrounding the electrode chamber, for example, in the manner shown at 20 in FIG. 1, with water entering the jacket at 19 and exiting at 21. However, the water jacket does not provide adequate cooling of the mercury vapor at the opposite end of the lamp. Furthermore, the use of a water jacket 20 around the base of the lamp 10 makes the bulkiest part of the lamp even bulkier.
As is apparent from the above, a need presently exists for improving the cooling systems of low pressure mercury vapor or gas discharge lamps to provide optimum cooling without adversely affecting the lamps' ability to generate high energy UV light or other radiation.